The invention pertains to a femoral head shaft prosthesis and, in particular, to a femoral head shaft prosthesis with a shaft portion which can be implanted into the marrow cavity of the femoral neck stump and can be attached in the marrow cavity by a cement layer surrounding the shaft portion.
A femoral head shaft prosthesis acts in a known manner together with a pin which can be screwed into the pelvic bone and which is threaded on the outside on a titanium outer dome. The outer dome is preferably conical in order to ensure increased support in addition to the contact with the threads. The inner cup of the pan is made of polyethylene which yields the best tribological values especially in combination with a ceramic finish of the femoral head.
Problems have arisen in the past, especially with respect to the primary stability and the long-term stability of the shaft portion in the marrow cavity of the upper thigh. Basically, two types of attachment are known. In a first process which used to be carried out regularly in the past, the shaft portion is attached by means of a cement combination in the marrow cavity of the upper thigh. The invention relates to a femoral head shaft prosthesis for this type of attachment which is still used today when it is necessary to put loads on the femoral head early, e.g. with older people.
The second process consists in cement-free implantation of a self-healing femoral head shaft prosthesis. A known method for this consists in coating the surface with hydroxyl apatite which induces the formation of new bone and promotes the incorporation of the prosthesis shaft through healing. Difficulties occur due to the fact that the marrow cavity in the femoral neck is physiologically different in each person, bends forward and to the outside to some extent and also varies in its cross-section. For this reason the known prosthesis shafts have been made in different shapes, and especially elliptical cross-sections and so-called physiological curves were produced.
With an implantation without cement all of these models have in common that the prosthesis shafts make contact only at points or over very small surfaces in the marrow cavity of the femoral neck so that incorporation through healing is delayed and impeded due to the initially unstable seating of the prothesis shaft.
To bring about an improvement in this matter, the implantation without cement of a femoral head shaft prosthesis has already become known (EP 0,359,097 Al) in which the lower portion of the shaft is cylindrical and can be implanted into a marrow cavity which has been drilled cylindrically to an equal diameter. Large contact surfaces with great primary stability and improved bone incorporation is thus achieved. The main problems with a cemented embodiment of a femoral head shaft prosthesis are basically different ones. To attach the prothesis shaft the latter is introduced with its relatively smooth surfaces into the marrow cavity which has been prepared with cement. A known embodiment (DE 32 47 726 Al) has a conical shaft tapering conically downward. The shaft bears here with portions of its circumference directly upon the inner wall of the marrow cavity so that direct contact is effected therein without intermediate cement layer. Due to the lack of a cement layer where direct contact occurs there is no bonding, so that the required strong fixing is reduced. A particular disadvantage is however especially due to the fact that the gap for the cement layer next to the points of direct contact first widens gradually so that the cement layer is very thin there and its layer thickness only increases at a further distance from the point of contact. This very thin cement layer in immediate proximity of the direct points of contact tends to come loose and to crumble because of its low internal stability. As a result the connection and the firm support are further reduced in the area adjoining the points of contact. Furthermore such detached cement particles have a toxic and destructive effect on the surrounding bone.
In order to achieve an improvement with respect to uniform cement layer thicknesses a method is already known by which a plastic star is pushed into the marrow cavity until it is in a position on which the underside of the prosthesis shaft will come to rest and by which the lower end of the prosthesis shaft is centered in the marrow cavity in this plastic star. For this purpose the prosthesis shaft is perforated and is set into the marrow cavity by means of a guide rod connected to the plastic star and passing through this perforation. Following the implantation the guide rod is removed. This process is obviously complicated and expensive, and despite the plastic star it cannot be ensured that the lower portion of the prosthesis shaft is seated securely and is centered in the receiving portion of the plastic star during the curing period of the cement. Also, the plastic star is a foreign body that remains disadvantageously permanently in the stump of the femoral neck. Ribbing and profiling of shaft surfaces are also known methods.
For an implantation without cement, relatively closely adjoining raised areas with small surfaces over the entire shaft surface are known and are intended to improve the anchoring of the prosthesis shaft (CH PS 675 826; DE-OS 32 16 539; DE-OS 34 15 934 and DE-GM 81 24 912). Such a surface structure with a plurality of relatively closely adjoining raised areas can provide advantages for the anchoring with an implantation without cement. When a prosthesis shaft is cemented in, such a surface structure is not suitable because the area of the cement layer would be constantly interrupted by many thinner areas and transitions so that the disadvantages described earlier in this context would occur at many locations.
Furthermore the structuring of the shaft surface by means of longitudinal grooves intended to achieve an improvement in cement bond between marrow cavity and shaft surface is also known (DE GM 88 11 758). These longitudinal grooves are however cut relatively deeply into the surface so that wide variations in the cement layer thickness are produced at the passage points between grooves and circumferences with a negative effect on stability.
It is the object of the instant invention to further develop a femoral head shaft prosthesis of this type so that improved primary and long-term stability can be achieved.